Hm, well I know bloom and material overlays don't appear in poster, but that looks like either a volumetric light or a corona (my money's on corona since you don't mention any volumetrics), but as far as I know these effects work in posters... I'm a test a thing or two...

Make sure you are not overriding your actual camera settings within the animation sets (Depth of Field, Motion blur) with override settings in your export options... that will cause it to look different from what you have in the viewport.

Another thing to consider..try rendering a movie image sequence instead of a poster.. then pull your still from the sequence. Like R234 pointed out, certain things don't work when exporting posters for some reason. You can opt to only render a very small sequence so you only are rendering the portion you want to create the poster from.

By the way... what are your camera/render settings anyway? As in.. Depth of Field, motion blur, etc. We are beginning to discover that high DoF settings can cause particle systems to not draw correctly during the render. For instance, at 256 DoF samples my smoke_cloud particle systems are completely see through and they certainly don't look like that in SFM's viewport. When i lower the DoF samples to 128 it gets a little better, but still not quite what they look like in my viewport. So anyway I wouldn't be surprised if high DoF was messing with other things like lighting in addition to particle systems.

Rendering a sequence allows you to choose from multiple shots of the same scene to get that "just right" one you're looking for as volumerics do change over time and do some of the other features that can't be set in a poster render..